01 / SOURCEWhat changes?
Traffic, combustion plant, industrial processes or agricultural emissions.
→02 / PATHWAYWhere does it travel?
Pollutants disperse in air; nitrogen can reach vegetation and soils through dry and wet deposition.
→03 / RECEPTORWhat is sensitive?
Identify the habitat, its designated features and the ecological evidence needed.
Concentrations: critical levels
Assess relevant gases, including NOₓ and ammonia (NH₃), against the applicable ecological critical levels. Airborne concentration is normally expressed in µg/m³.
Deposition: critical loads
Assess nitrogen arriving at the habitat against its nutrient-nitrogen critical load, usually in kg N/ha/year. Consider acid deposition separately where relevant. The appropriate value depends on the habitat.
Concentration and deposition are different measures: a human-health NO₂ result does not answer the ecological question. APIS explains the distinction →
WHEN HRA ENTERS THE BRIEF
Evidence for a planning decision.
For a protected habitats site, screening considers likely significant effects alone and in combination with other plans or projects. Where these cannot be excluded, the competent authority must undertake an appropriate assessment.
The applicant supplies supporting evidence. ACP’s air quality work can inform that evidence alongside ecological interpretation; the authority makes the statutory HRA decision. Read the national HRA guidance →
What the assessment should connect
- The emissions source and affected roads or plant.
- Receptor locations, sensitive features and conservation objectives.
- Background conditions, project contributions and relevant combined effects.
- Model assumptions, uncertainty and any mitigation evidence.
Natural England’s standing advice covers specified development types and protected sites in England. Its scope excludes local plans and nationally significant infrastructure projects. SSSI protection and HRA are distinct assessment routes. Check the applicable advice →
Local examples: check the actual requirement
These local examples illustrate different planning circumstances. They are not an exhaustive England-wide HRA audit, and do not impose a blanket assessment requirement.
Cornwall Council validation update
The council flags protected-site air pollution advice for traffic-generating, agricultural and industrial proposals. It identifies circumstances needing detailed assessment and says its validation guide will be updated.
Use the current update alongside the application checklist; do not infer that every proposal requires a detailed model.
Read the council source →Epping Forest Applicant guidance, January 2025 update
The council identifies vehicle-related atmospheric pollution as a pathway affecting Epping Forest SAC. Its guidance explains when project-specific HRA information may be needed.
Check the proposal against the modelled scenario and mitigation relied upon. The recreational zone is not a universal air-pollution screening distance.
Read the council source →Waltham Forest Published applicant requirements
The council asks applications creating one or more new homes to include project-level HRA screening and appropriate-assessment information, with forms for sites inside and outside 400 metres.
Follow the council template. This HRA requirement alone does not establish a requirement for detailed air dispersion modelling.
Read the council source →Redbridge Published Epping Forest SAC guidance
The council identifies recreation and vehicle-related air pollution as separate concerns, and sets out HRA arrangements for qualifying residential proposals and prior approvals.
Check the mapped zone, proposal type and applicable procedure. Recreational contributions should not be described as air-pollution mitigation.
Read the council source →Wealden / Ashdown Forest Published evidence base
The council links traffic, air quality and nitrogen deposition to Ashdown Forest SAC and publishes monitoring and modelling evidence.
This is evidence of a relevant pathway, not a current blanket validation trigger. Confirm the project-specific scope with the council.
Read the council source →New Forest National Park Historic plan HRA: context only
The 2018 plan assessment examines NOx, ammonia and deposition with ecological interpretation of sensitive habitats.
Historic strategic evidence does not set today’s project threshold. Confirm current application requirements and any reliance on strategic evidence.
Read the council source →
Sources checked 20 October 2026. Local requirements, source status and project-specific scope must be confirmed when commissioning.